House Frey was a vassal of House Tully of the Riverlands and holds its fealty to the Lord of Riverrun. Its current ruler is Lord Walder Frey. Their castle is known as the Twins, for its two identical keeps on either side of a river linked by a bridge, sometimes called the Stone Bridge. The Twins is one of the primary crossings over the Green Fork of the Trident River, and tolls from bridge crossings have made the Freys quite wealthy fairly quickly, elevating them from obscurity six centuries ago to being one of the most powerful noble Houses in the Riverlands - though they are looked down upon by older aristocratic families.

The Heraldry of House Frey represents their castle-seat of the Twins: it consists of two grey towers linked by a bridge, on a darker grey background, over an escutcheon of blue water.[1]

During Robert's Rebellion, the jest was made that Lord Walder arrived late to the Battle of the Trident because he was waiting for his army to come of age...given that he had produced most of his soldiers from within his own britches (directly or indirectly through his previous children). Walder devoted the next seventeen years to making this claim a reality.

Walder Rivers, called "Black Walder," Lord Walder's bastard son. The most prominent of Lord Walder's bastards, Black Walder Rivers has risen to a position of prominence within House Frey and is one of his father's chief enforcers.

In the A Song of Ice and Fire novels, House Frey controls the Twins, two castles that hold the Crossing, the only bridge over the Green Fork of the Trident for hundreds of miles in either direction. House Frey is the northern-most house of the Riverlands, not far south of the loose border with the North, and has a history of enmity with House Reed, who control the marshes of the Neck to the north of the Twins.

House Frey was a minor house of no notability until they bridged the Green Fork six centuries ago and defending the bridge with two wooden castles. Charging wayfarers for the use of the Crossing, they grew rich and influential, replacing the castles with strong stone keeps. Their growing wealth and influence saw them gain several vassal houses of their own: House Erenford, House Haigh, and House Charlton. Given the relatively recent rise of their House, the Freys are looked down upon by other noble Houses of the Riverlands as uncouth and honorless nouveau riche. Good nutrition has given them some length of bone, but they're no more than a few generations removed from poverty.

Their military strength is formidable, allowing them to raise four thousand troops by themselves - almost 10% of the total strength of the Riverlands. The current Lord Frey's father was involved in the scandal of the Whitewalls Tourney, when the Second Blackfyre Rebellion was halted in its tracks, roughly ninety years ago, leading to the current Lord Walder Frey being a cautious man, extremely reluctant to commit himself to any cause until the outcome is certain. During Robert's Rebellion he delayed the arrival of his army at the Battle of the Trident until Robert Baratheon had secured victory. For this, Lord Frey's liege, Hoster Tully, dubbed him "The Late Lord Frey", a nickname which has stuck, to Walder Frey's fury.

Little has been revealed about members of House Frey who are not descendants of Lord Walder himself. It is probable that due to his advanced age of ninety years, Walder may have simply outlived any siblings or cousins he may have had. The one minor reference to Walder's family is in the Tales of Dunk and Egg prequel novellas, which take place almost ninety years before the beginning of A Game of Thrones. Walder himself actually makes a brief appearance in the prequels as a misbehaving toddler - making him one of the only characters who are present in both the main series and the prequel novellas; Maester Aemon is about a decade older than Walder, thus he is also alive during the early prequel novellas, though he does not prominently appear (instead, his younger brother Aegon "Egg" Targaryen is a major character). Walder's father appears in the prequel but is referred to simply as "Lord Frey" without revealing what his name was. However, the prequels also reveal that Walder actually had a sister, who was about a dozen years older than him, who marries Lord Ambrose of House Butterwell, a noble House from the Crownlands. Given that the Butterwells are disgraced during the events of the prequels, it isn't clear if they died out between the prequels and the main series - no current Butterwells have been mentioned in the main series. Given the limited information, it isn't clear if Lord Walder has any surviving nieces, nephews, grand-nephews, or distant cousins from cadet branches of the main Frey line.

Despite their prominence in the storyline, the motto of House Frey has not yet been revealed, even in the first five novels.

The Heraldry of House Frey is somewhat different in the books than it is in the TV series. In the books, it is the two towers and bridge of the Twins colored blue, on a silver-grey background. The TV series made the towers stone grey, and over the same dark grey field as in the books, but now above a blue escutcheon which is drawn to resemble the waves of the Green Fork of the Trident River. The color change may be because after the blue river was added as an escutcheon at the bottom, it would have been visually confusing to have the castle be the same color as the water. Unfortunately, reversing the colors like this makes it vaguely resemble the reversed colors used in heraldry by bastard children (though in such cases, without the escutcheon, the entire background field is blue, not just the escutcheon).